Description of the Prior Art
Polycrystalline silicon rods are used in the manufacture of single crystal rods for the semiconductor industry by either the float-zone melting process or by the Czochralski crystal-pulling technique. The single crystal rods are further processed to form silicon wafers from which semiconductor silicon chips are made.
Polycrystalline rods are made by the pyrolytic decomposition of a gaseous silicon compound, such as silane, on a heated rod shaped silicon filament or, alternatively, from a high melting point metal having good electrical conductivity, such as tungsten or tantalum. The principles of the design of present state-of the-art reactors for the pyrolysis of silane are set forth in, for example, U.S. Pat. Nos. 3,147,141, 4,147,814, 4,150,168.
The pyrolysis of silane to form silicon and hydrogen is performed in a reactor housing a plurality of silicon filament rods. The silicon filament seed rods are electrically heated until red hot to react with silane gas fed slowly into the reactor. The process is started with the silicon filament seed rod at ambient temperature. Since pure silicon has very high electrical resistivity, the seed rod is doped with impurities to lower its electrical resistivity, thereby facilitating the start of the heating process.
The build up of polycrystalline silicon is the result of the heterogeneous decomposition of silane on the glowing hot surface of the silicon filament seed rod. The reaction deposits silicon on the surface of the filament rod and releases hydrogen gas which flows by the natural thermal convective transport of the resulting silane/hydrogen mixture in the reactor. This natural thermal convection is created by the hydrogen being heated by the hot silicon rod causing it to rise at a modest velocity estimated at about 5 feet/sec and then cooled by the reactor walls whereupon it flows downward at a reduced velocity.
The silane pyrolysis proceeds at a rate determined by the concentration of monosilane in its gaseous phase and local temperature. Desirable formation of dense crystalline silicon occurs in a narrow boundary layer surrounding the hot silicon rod. To the extent that the bulk gas temperature is high enough to cause pyrolysis, a competing homogeneous silane decomposition reaction takes place wherein deleterious submicron powder is formed. In practice, it is found that this powder formation increases with increasing rod diameter.
Although much of the powder particles are deposited on the cold reactor walls by thermophoretic decomposition, the number of powder particles eventually build up to intolerable levels. Also, some of this powder ultimately deposits directly onto the silicon rod or, alternatively, flakes of deposited powder fall from the reactor walls onto the product. This affects both the surface morphology of the rod and causes undesirable contamination due the incompatible melting behavior of the powdered silicon during subsequent single crystal formation techniques. If the powder concentration is sufficiently high, the surface morphology and/or the processability of the resulting silicon rod deteriorates to the point where the product becomes totally unacceptable.
Accordingly, the formation of silicon powder by homogeneous decomposition effectively places an upper limit on the diameter and/or growth rate of the rods that may be grown during any production run, although larger rods and/or higher growth rates would clearly be more cost effective. Once the amount of powder deposited on the reactor walls is such that it appears imminent that it is about to flake off, the reaction must be stopped so as to prevent contamination. So too, while it is known that increasing the silane concentration within the reactor will effect a higher growth rate of the silicon rod, the concomitant production of silicon powder negates any advantage that such increased growth rate may have.
This limited capability of the reactors translates into low productivity per reactor, a large consumption of electrical power per unit of production and a large capital investment per annual unit of capacity. While the fraction of silane decomposing homogeneously into fine powder is small compared to the fraction decomposing heterogeneously into useful product, it is clear that the formation of even this relatively small amount of powder is quite detrimental and necessitates a need for finding a way to improve reactor productivity and efficiency.
In a copending application U.S. Ser. No. 062,256 commonly assigned to the same assignee, it was discovered that by discharging the gases generated in the reactor and recycling the exhaust gas back into the reactor at a rate high enough to entrain silicon powder, the silicon powder may be filtered out from the external loop before the exhaust gas is returned to the reactor. This permits more silane to be introduced in the reactor to increase the silane decomposition without increasing the concentration of silicon powder in the reactor.